Recently, a bi-directional communication through a packet communication network is actively performed as information technology is developed. In the packet communication network, communication data is packetized and transmitted based on an Internet standard protocol such as transmission control protocol/Internet protocol (TCP/IP). See, for example, Japanese Laid-open Patent Publication No. 2010-154201. As the amount of data communication through the packet communication network increases, it becomes more important for a service provider who provides a data communication service to monitor and manage the quality of data communication.